1. Field of the Invention
The present invention generally relates to free-space optical communications systems, and, more specifically, to a method and apparatus for improving transmission of free-space optical communication signals through windows with surface imperfections, such as pitting, surface waviness, and other non-flat surface distortions.
2. Background Information
With the increasing popularity of wide area networks (WANs), such as the Internet and/or the World Wide Web, network growth and traffic has exploded in recent years. Network users continue to demand faster networks and more access for both businesses and consumers. As network demands continue to increase, existing network infrastructures and technologies are reaching their limits.
An alternative to present day hardwired or fiber network solutions is the use of wireless optical communications. Wireless optical communications utilize point-to-point communications through free-space and therefore do not require the routing of cables or fibers between locations. Thus, wireless optical communications are also known as free-space or atmospheric optical communications. For instance, in a free-space optical communication system, a beam of light is directed through free-space from a transmitter at a first location to a receiver at a second location. Data or information is encoded into the beam of light, and therefore, the information is transmitted through free-space from the first location to the second location.
A conventional free-space optical system is shown in FIGS. 1A and 1B. The free-space optical system includes a pair of transceiver stations 10 that are typically located on or in separate buildings or towers, such as depicted by buildings 11 and 12. Each transceiver station 10 includes a primary collector 13 to which a secondary mirror 14 is coupled via a plurality of rigid struts 16. The transceiver stations further include a transmitted signal lens 18 mounted within secondary mirror 14, and a set of transmitter/receiver optics and electronics 20. All of components 13, 14, 16, 18, and 20 are operatively coupled to a yoke that is connected to a base 22 via a gimble assembly, such that these components are all moved in response to a gimbled movement of the yoke relative to a static surface on which the base 22 is placed.
With reference to FIG. 1B, data is transmitted from a transceiver station 10T to a transceiver station 10R in the following manner. An optical signal 24 is generated by transmitter/receiver optics and electronics 20T of transceiver station 10T and directed through an opening 26T defined in primary collector 13T towards transmitted signal lens 18T, which produces a collimated signal 28. As collimated signal 28 moves toward transceiver station 10R, the width of the signal diverges very gradually. As will be recognized by those skilled in the art, the divergence of the various optical signals depicted in the Figures contained herein are exaggerated for clarity. Upon reaching transceiver station 10R, the outer portions of collimated signal 28 impinge upon primary collector 13R, which comprises a concave mirrored surface that redirects those portions of the signal that impinge upon it toward secondary mirror 14R. Collimated signal 228 is then reflected by secondary mirror 14R towards the secondary mirror's focal point 30, where it is received by transmitter/receiver optics and electronics 20R.